Macroblock Level Adaptive Frame/Field Coding for Digital Video Content

ABSTRACT

A method and system of encoding and decoding digital video content. The digital video content comprises a stream of pictures which can each be intra, predicted, or bi-predicted pictures. Each of the pictures comprises macroblocks that can be further divided into smaller blocks. The method entails encoding and decoding each of the smaller blocks in each picture in said stream of pictures in either frame mode or in field mode.

The present application claims priority under 35 U.S.C. 119(e) from thefollowing previously filed Provisional Patent Application Ser. No.60/333,921, filed Nov. 27, 2001; Ser. No. 60/395,734, filed Jul. 12,2002; Ser. No. 60/398,161, filed Jul. 23, 2002, all of which are hereinincorporated by reference. This application also is a Continuation ofand claims the benefit of U.S. Ser. No. 11/027,869, filed Dec. 30, 2004,which is a Divisional of U.S. patent application Ser. No. 10/301,290filed on Nov. 20, 2002, both of which are herein incorporated byreference.

TECHNICAL FIELD

The present invention relates to encoding and decoding of digital videocontent. More specifically, the present invention relates to frame modeand field mode encoding of digital video content at a macroblock levelas used in the MPEG-4 Part 10 AVC/H.264 standard video coding standard.

BACKGROUND

Video compression is used in many current and emerging products. It isat the heart of digital television set-top boxes (STBs), digitalsatellite systems (DSSs), high definition television (HDTV) decoders,digital versatile disk (DVD) players, video conferencing, Internet videoand multimedia content, and other digital video applications. Withoutvideo compression, digital video content can be extremely large, makingit difficult or even impossible for the digital video content to beefficiently stored, transmitted, or viewed.

The digital video content comprises a stream of pictures that can bedisplayed as an image on a television receiver, computer monitor, orsome other electronic device capable of displaying digital videocontent. A picture that is displayed in time before a particular pictureis in the “backward direction” in relation to the particular picture.Likewise, a picture that is displayed in time after a particular pictureis in the “forward direction” in relation to the particular picture.

Video compression is accomplished in a video encoding, or coding,process in which each picture is encoded as either a frame or as twofields. Each frame comprises a number of lines of spatial information.For example, a typical frame contains 480 horizontal lines. Each fieldcontains half the number of lines in the frame. For example, if theframe comprises 480 horizontal lines, each field comprises 240horizontal lines. In a typical configuration, one of the fieldscomprises the odd numbered lines in the frame and the other fieldcomprises the even numbered lines in the frame. The field that comprisesthe odd numbered lines will be referred to as the “top” field hereafterand in the appended claims, unless otherwise specifically denoted.Likewise, the field that comprises the even numbered lines will bereferred to as the “bottom” field hereafter and in the appended claims,unless otherwise specifically denoted. The two fields can be interlacedtogether to form an interlaced frame.

The general idea behind video coding is to remove data from the digitalvideo content that is “non-essential.” The decreased amount of data thenrequires less bandwidth for broadcast or transmission. After thecompressed video data has been transmitted, it must be decoded, ordecompressed. In this process, the transmitted video data is processedto generate approximation data that is substituted into the video datato replace the “non-essential” data that was removed in the codingprocess.

Video coding transforms the digital video content into a compressed formthat can be stored using less space and transmitted using less bandwidththan uncompressed digital video content. It does so by taking advantageof temporal and spatial redundancies in the pictures of the videocontent. The digital video content can be stored in a storage mediumsuch as a hard drive, DVD, or some other non-volatile storage unit.

There are numerous video coding methods that compress the digital videocontent. Consequently, video coding standards have been developed tostandardize the various video coding methods so that the compresseddigital video content is rendered in formats that a majority of videoencoders and decoders can recognize. For example, the Motion PictureExperts Group (MPEG) and International Telecommunication Union (ITU-T)have developed video coding standards that are in wide use. Examples ofthese standards include the MPEG-1, MPEG-2, MPEG-4, ITU-T H261, andITU-T H263 standards.

Most modern video coding standards, such as those developed by MPEG andITU-T, are based in part on a temporal prediction with motioncompensation (MC) algorithm. Temporal prediction with motioncompensation is used to remove temporal redundancy between successivepictures in a digital video broadcast.

The temporal prediction with motion compensation algorithm typicallyutilizes one or two reference pictures to encode a particular picture. Areference picture is a picture that has already been encoded. Bycomparing the particular picture that is to be encoded with one of thereference pictures, the temporal prediction with motion compensationalgorithm can take advantage of the temporal redundancy that existsbetween the reference picture and the particular picture that is to beencoded and encode the picture with a higher amount of compression thanif the picture were encoded without using the temporal prediction withmotion compensation algorithm. One of the reference pictures may be inthe backward direction in relation to the particular picture that is tobe encoded. The other reference picture is in the forward direction inrelation to the particular picture that is to be encoded.

However, as the demand for higher resolutions, more complex graphicalcontent, and faster transmission time increases, so does the need forbetter video compression methods. To this end, a new video codingstandard is currently being developed jointly by ISO and ITU-T. This newvideo coding standard is called the MPEG-4 Advanced Video Coding(AVC)/H.264 standard.

SUMMARY OF THE INVENTION

In one of many possible embodiments, the present invention provides amethod of encoding, decoding, and bitstream generation of digital videocontent. The digital video content comprises a stream of pictures whichcan each be intra, predicted, or bi-predicted pictures. Each of thepictures comprises macroblocks that can be further divided into smallerblocks. The method entails encoding and decoding each of the macroblocksin each picture in said stream of pictures in either frame mode or infield mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the presentinvention and are a part of the specification. Together with thefollowing description, the drawings demonstrate and explain theprinciples of the present invention. The illustrated embodiments areexamples of the present invention and do not limit the scope of theinvention.

FIG. 1 illustrates an exemplary sequence of three types of pictures thatcan be used to implement the present invention, as defined by anexemplary video coding standard such as the MPEG-4 Part 10 AVC/H.264standard.

FIG. 2 shows that each picture is preferably divided into slicescontaining macroblocks according to an embodiment of the presentinvention.

FIG. 3 a shows that a macroblock can be further divided into a blocksize of 16 by 8 pixels according to an embodiment of the presentinvention.

FIG. 3 b shows that a macroblock can be further divided into a blocksize of 8 by 16 pixels according to an embodiment of the presentinvention.

FIG. 3 c shows that a macroblock can be further divided into a blocksize of 8 by 8 pixels according to an embodiment of the presentinvention.

FIG. 3 d shows that a macroblock can be further divided into a blocksize of 8 by 4 pixels according to an embodiment of the presentinvention.

FIG. 3 e shows that a macroblock can be further divided into a blocksize of 4 by 8 pixels according to an embodiment of the presentinvention.

FIG. 3 f shows that a macroblock can be further divided into a blocksize of 4 by 4 pixels according to an embodiment of the presentinvention.

FIG. 4 shows a picture construction example using temporal predictionwith motion compensation that illustrates an embodiment of the presentinvention.

FIG. 5 shows that a macroblock is split into a top field and a bottomfield if it is to be encoded in field mode.

FIG. 6 a shows that a macroblock that is encoded in field mode can bedivided into a block with a size of 16 by 8 pixels according to anembodiment of the present invention.

FIG. 6 b shows that a macroblock that is encoded in field mode can bedivided into a block with a size of 8 by 8 pixels according to anembodiment of the present invention.

FIG. 6 c shows that a macroblock that is encoded in field mode can bedivided into a block with a size of 4 by 8 pixels according to anembodiment of the present invention.

FIG. 6 d shows that a macroblock that is encoded in field mode can bedivided into a block with a size of 4 by 4 pixels according to anembodiment of the present invention.

FIG. 7 illustrates an exemplary pair of macroblocks that can be used inAFF coding on a pair of macroblocks according to an embodiment of thepresent invention.

FIG. 8 shows that a pair of macroblocks that is to be encoded in fieldmode is first split into one top field 16 by 16 pixel block and onebottom field 16 by 16 pixel block.

FIG. 9 shows two possible scanning paths in AFF coding of pairs ofmacroblocks.

FIG. 10 illustrates another embodiment of the present invention whichextends the concept of AFF coding on a pair of macroblocks to AFF codingto a group of four or more neighboring macroblocks.

FIG. 11 shows some of the information included in the bitstream whichcontains information pertinent to each macroblock within a stream.

FIG. 12 shows a block that is to be encoded and its neighboring blocksand will be used to explain various preferable methods of calculatingthe PMV of a block in a macroblock.

FIG. 13 shows an alternate definition of neighboring blocks if thescanning path is a vertical scanning path.

FIG. 14 shows that each pixel value is predicted from neighboringblocks' pixel values according to an embodiment of the presentinvention.

FIG. 15 shows different prediction directions for intra_(—)4×4 coding.

FIGS. 16 a-b illustrate that the chosen intra-prediction mode(intra_pred_mode) of a 4 by 4 pixel block is highly correlated with theprediction modes of adjacent blocks.

FIGS. 17 a-d show neighboring blocks definitions in relation to acurrent macroblock pair that is to be encoded.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention provides a method of adaptive frame/field (AFF)coding of digital video content comprising a stream of pictures orslices of a picture at a macroblock level. The present invention extendsthe concept of picture level AFF to macroblocks. In AFF coding at apicture level, each picture in a stream of pictures that is to beencoded is encoded in either frame mode or in field mode, regardless ofthe frame or field coding mode of other pictures that are to be coded.If a picture is encoded in frame mode, the two fields that make up aninterlaced frame are coded jointly. Conversely, if a picture is encodedin field mode, the two fields that make up an interlaced frame are codedseparately. The encoder determines which type of coding, frame modecoding or field mode coding, is more advantageous for each picture andchooses that type of encoding for the picture. The exact method ofchoosing between frame mode and field mode is not critical to thepresent invention and will not be detailed herein.

As noted above, the MPEG-4 Part 10 AVC/H.264 standard is a new standardfor encoding and compressing digital video content. The documentsestablishing the MPEG-4 Part 10 AVC/H.264 standard are herebyincorporated by reference, including “Joint Final Committee Draft (JFCD)of Joint Video Specification” issued by the Joint Video Team (JVT) onAug. 10, 2002. (ITU-T Rec. H.264 & ISO/IEC 14496-10 AVC). The JVTconsists of experts from ISO or MPEG and ITU-T. Due to the public natureof the MPEG-4 Part 10 AVC/H.264 standard, the present specification willnot attempt to document all the existing aspects of MPEG-4 Part 10AVC/H.264 video coding, relying instead on the incorporatedspecifications of the standard.

this method of AFF encoding is compatible with and will be explainedusing the MPEG-4 Part 10 AVC/H.264 standard guidelines, it can bemodified and used as best serves a particular standard or application.

Using the drawings, the preferred embodiments of the present inventionwill now be explained.

FIG. 1 illustrates an exemplary sequence of three types of pictures thatcan be used to implement the present invention, as defined by anexemplary video coding standard such as the MPEG-4 Part 10 AVC/H.264standard. As previously mentioned, the encoder encodes the pictures andthe decoder decodes the pictures. The encoder or decoder can be aprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), coder/decoder (CODEC), digital signalprocessor (DSP), or some other electronic device that is capable ofencoding the stream of pictures. However, as used hereafter and in theappended claims, unless otherwise specifically denoted, the term“encoder” will be used to refer expansively to all electronic devicesthat encode digital video content comprising a stream of pictures. Theterm “decoder” will be used to refer expansively to all electronicdevices that decode digital video content comprising a stream ofpictures.

As shown in FIG. 1, there are preferably three types of pictures thatcan be used in the video coding method. Three types of pictures aredefined to support random access to stored digital video content whileexploring the maximum redundancy reduction using temporal predictionwith motion compensation. The three types of pictures are intra (I)pictures (100), predicted (P) pictures (102 a,b), and bi-predicted (B)pictures (101 a-d). An I picture (100) provides an access point forrandom access to stored digital video content and can be encoded onlywith slight compression. Intra pictures (100) are encoded withoutreferring to reference pictures.

A predicted picture (102 a,b) is encoded using an I, P, or B picturethat has already been encoded as a reference picture. The referencepicture can be in either the forward or backward temporal direction inrelation to the P picture that is being encoded. The predicted pictures(102 a,b) can be encoded with more compression than the intra pictures(100).

A bi-predicted picture (101 a-d) is encoded using two temporal referencepictures: a forward reference picture and a backward reference picture.The forward reference picture is sometimes called a past referencepicture and the backward reference picture is sometimes called a futurereference picture. An embodiment of the present invention is that theforward reference picture and backward reference picture can be in thesame temporal direction in relation to the B picture that is beingencoded. Bi-predicted pictures (101 a-d) can be encoded with the mostcompression out of the three picture types.

Reference relationships (103) between the three picture types areillustrated in FIG. 1. For example, the P picture (102 a) can be encodedusing the encoded I picture (100) as its reference picture. The Bpictures (101 a-d) can be encoded using the encoded I picture (100) orthe encoded P picture (102 a) as its reference pictures, as shown inFIG. 1. Under the principles of an embodiment of the present invention,encoded B pictures (101 a-d) can also be used as reference pictures forother B pictures that are to be encoded. For example, the B picture (101c) of FIG. 1 is shown with two other B pictures (101 b and 101 d) as itsreference pictures.

The number and particular order of the I (100), B (101 a-d), and P (102a,b) pictures shown in FIG. 1 are given as an exemplary configuration ofpictures, but are not necessary to implement the present invention. Anynumber of I, B, and P pictures can be used in any order to best serve aparticular application. The MPEG-4 Part 10 AVC/H.264 standard does notimpose any limit to the number of B pictures between two referencepictures nor does it limit the number of pictures between two Ipictures.

FIG. 2 shows that each picture (200) is preferably divided into slices(202). A slice (202) comprises a group of macroblocks (201). Amacroblock (201) is a rectangular group of pixels. As shown in FIG. 2, apreferable macroblock (201) size is 16 by 16 pixels.

FIGS. 3 a-f show that a macroblock can be further divided into smallersized blocks. For example, as shown in FIGS. 3 a-f, a macroblock can befurther divided into block sizes of 16 by 8 pixels (FIG. 3 a; 300), 8 by16 pixels (FIG. 3 b; 301), 8 by 8 pixels (FIG. 3 c; 302), 8 by 4 pixels(FIG. 3 d; 303), 4 by 8 pixels (FIG. 3 e; 304), or 4 by 4 pixels (FIG. 3f; 305). These smaller block sizes are preferable in some applicationsthat use the temporal prediction with motion compensation algorithm.

FIG. 4 shows a picture construction example using temporal predictionwith motion compensation that illustrates an embodiment of the presentinvention. Temporal prediction with motion compensation assumes that acurrent picture, picture N (400), can be locally modeled as atranslation of another picture, picture N−1 (401). The picture N−1 (401)is the reference picture for the encoding of picture N (400) and can bein the forward or backwards temporal direction in relation to picture N(400).

As shown in FIG. 4, each picture is preferably divided into slicescontaining macroblocks (201 a,b). The picture N−1 (401) contains animage (403) that is to be shown in picture N (400). The image (403) willbe in a different temporal position in picture N (402) than it is inpicture N−1 (401), as shown in FIG. 4. The image content of eachmacroblock (201 b) of picture N (400) is predicted from the imagecontent of each corresponding macroblock (201 a) of picture N−1 (401) byestimating the required amount of temporal motion of the image contentof each macroblock (201 a) of picture N−1 (401) for the image (403) tomove to its new temporal position (402) in picture N (400). Instead ofthe original image (402) being encoded, the difference (404) between theimage (402) and its prediction (403) is actually encoded andtransmitted.

For each image (402) in picture N (400), the temporal prediction canoften be described by motion vectors that represent the amount oftemporal motion required for the image (403) to move to a new temporalposition in the picture N (402). The motion vectors (406) used for thetemporal prediction with motion compensation need to be encoded andtransmitted.

FIG. 4 shows that the image (402) in picture N (400) can be representedby the difference (404) between the image and its prediction and theassociated motion vectors (406). The exact method of encoding using themotion vectors can vary as best serves a particular application and canbe easily implemented by someone who is skilled in the art.

To understand macroblock level AFF coding, a brief overview of picturelevel AFF coding of a stream of pictures will now be given. A frame ofan interlaced sequence contains two fields, the top field and the bottomfield, which are interleaved and separated in time by a field period.The field period is half the time of a frame period. In picture levelAFF coding, the two fields of an interlaced frame can be coded jointlyor separately. If they are coded jointly, frame mode coding is used.Conversely, if the two fields are coded separately, field mode coding isused.

Fixed frame/field coding, on the other hand, codes all the pictures in astream of pictures in one mode only. That mode can be frame mode or itcan be field mode. Picture level AFF is preferable to fixed frame/fieldcoding in many applications because it allows the encoder to chose whichmode, frame mode or field mode, to encode each picture in the stream ofpictures based on the contents of the digital video material. AFF codingresults in better compression than does fixed frame/field coding in manyapplications.

An embodiment of the present invention is that AFF coding can beperformed on smaller portions of a picture. This small portion can be amacroblock, a pair of macroblocks, or a group of macroblocks. Eachmacroblock, pair of macroblocks, or group of macroblocks or slice isencoded in frame mode or in field mode, regardless of how the othermacroblocks in the picture are encoded. AFF coding in each of the threecases will be described in detail below.

In the first case, AFF coding is performed on a single macroblock. Ifthe macroblock is to be encoded in frame mode, the two fields in themacroblock are encoded jointly. Once encoded as a frame, the macroblockcan be further divided into the smaller blocks of FIGS. 3 a-f for use inthe temporal prediction with motion compensation algorithm.

However, if the macroblock is to be encoded in field mode, themacroblock (500) is split into a top field (501) and a bottom field(502), as shown in FIG. 5. The two fields are then coded separately. InFIG. 5, the macroblock has M rows of pixels and N columns of pixels. Apreferable value of N and M is 16, making the macroblock (500) a 16 by16 pixel macroblock. As shown in FIG. 5, every other row of pixels isshaded. The shaded areas represent the rows of pixels in the top fieldof the macroblock (500) and the unshaded areas represent the rows ofpixels in the bottom field of the macroblock (500).

As shown in FIGS. 6 a-d, a macroblock that is encoded in field mode canbe divided into four additional blocks. A block is required to have asingle parity. The single parity requirement is that a block cannotcomprise both top and bottom fields. Rather, it must contain a singleparity of field. Thus, as shown in FIGS. 6 a-d, a field mode macroblockcan be divided into blocks of 16 by 8 pixels (FIG. 6 a; 600), 8 by 8pixels (FIG. 6 b; 601), 4 by 8 pixels (FIGS. 6 c; 602), and 4 by 4pixels (FIG. 6 d; 603). FIGS. 6 a-d show that each block contains fieldsof a single parity.

AFF coding on macroblock pairs will now be explained. AFF coding onmacroblock pairs will be occasionally referred to as pair based AFFcoding. A comparison of the block sizes in FIGS. 6 a-d and in FIGS. 3a-f show that a macroblock encoded in field mode can be divided intofewer block patterns than can a macroblock encoded in frame mode. Theblock sizes of 16 by 16 pixels, 8 by 16 pixels, and 8 by 4 pixels arenot available for a macroblock encoded in field mode because of thesingle parity requirement. This implies that the performance of singlemacroblock based AFF may not be good for some sequences or applicationsthat strongly favor field mode coding. In order to guarantee theperformance of field mode macroblock coding, it is preferable in someapplications for macroblocks that are coded in field mode to have thesame block sizes as macroblocks that are coded in frame mode. This canbe achieved by performing AFF coding on macroblock pairs instead of onsingle macroblocks.

FIG. 7 illustrates an exemplary pair of macroblocks (700) that can beused in AFF coding on a pair of macroblocks according to an embodimentof the present invention. If the pair of macroblocks (700) is to beencoded in frame mode, the pair is coded as two frame-based macroblocks.In each macroblock, the two fields in each of the macroblocks areencoded jointly. Once encoded as frames, the macroblocks can be furtherdivided into the smaller blocks of FIGS. 3 a-f for use in the temporalprediction with motion compensation algorithm.

However, if the pair of macroblocks (700) is to be encoded in fieldmode, it is first split into one top field 16 by 16 pixel block (800)and one bottom field 16 by 16 pixel block (801), as shown in FIG. 8. Thetwo fields are then coded separately. In FIG. 8, each macroblock in thepair of macroblocks (700) has N=16 columns of pixels and M=16 rows ofpixels. Thus, the dimensions of the pair of macroblocks (700) is 16 by32 pixels. As shown in FIG. 8, every other row of pixels is shaded. Theshaded areas represent the rows of pixels in the top field of themacroblocks and the unshaded areas represent the rows of pixels in thebottom field of the macroblocks. The top field block (800) and thebottom field block (801) can now be divided into one of the possibleblock sizes of FIGS. 3 a-f.

According to an embodiment of the present invention, in the AFF codingof pairs of macroblocks (700), there are two possible scanning paths. Ascanning path determines the order in which the pairs of macroblocks ofa picture are encoded. FIG. 9 shows the two possible scanning paths inAFF coding of pairs of macroblocks (700). One of the scanning paths is ahorizontal scanning path (900). In the horizontal scanning path (900),the macroblock pairs (700) of a picture (200) are coded from left toright and from top to bottom, as shown in FIG. 9. The other scanningpath is a vertical scanning path (901). In the vertical scanning path(901), the macroblock pairs (700) of a picture (200) are coded from topto bottom and from left to right, as shown in FIG. 9. For frame modecoding, the top macroblock of a macroblock pair (700) is coded first,followed by the bottom macroblock. For field mode coding, the top fieldmacroblock of a macroblock pair is coded first followed by the bottomfield macroblock.

Another embodiment of the present invention extends the concept of AFFcoding on a pair of macroblocks to AFF coding on a group of four or moreneighboring macroblocks (902), as shown in FIG. 10. AFF coding on agroup of macroblocks will be occasionally referred to as group based AFFcoding. The same scanning paths, horizontal (900) and vertical (901), asare used in the scanning of macroblock pairs are used in the scanning ofgroups of neighboring macroblocks (902). Although the example shown inFIG. 10 shows a group of four macroblocks, the group can be more thanfour macroblocks.

If the group of macroblocks (902) is to be encoded in frame mode, thegroup coded as four frame-based macroblocks. In each macroblock, the twofields in each of the macroblocks are encoded jointly. Once encoded asframes, the macroblocks can be further divided into the smaller blocksof FIGS. 3 a-f for use in the temporal prediction with motioncompensation algorithm.

However, if a group of four macroblocks (902), for example, is to beencoded in field mode, it is first split into one top field 32 by 16pixel block and one bottom field 32 by 16 pixel block. The two fieldsare then coded separately. The top field block and the bottom fieldblock can now be divided into macroblocks. Each macroblock is furtherdivided into one of the possible block sizes of FIGS. 3 a-f. Becausethis process is similar to that of FIG. 8, a separate figure is notprovided to illustrate this embodiment.

In AFF coding at the macroblock level, a frame/field flag bit ispreferably included in a picture's bitstream to indicate which mode,frame mode or field mode, is used in the encoding of each macroblock.The bitstream includes information pertinent to each macroblock within astream, as shown in FIG. 11. For example, the bitstream can include apicture header (110), run information (111), and macroblock type (113)information. The frame/field flag (112) is preferably included beforeeach macroblock in the bitstream if AFF is performed on each individualmacroblock. If the AFF is performed on pairs of macroblocks, theframe/field flag (112) is preferably included before each pair ofmacroblock in the bitstream. Finally, if the AFF is performed on a groupof macroblocks, the frame/field flag (112) is preferably included beforeeach group of macroblocks in the bitstream. One embodiment is that theframe/field flag (112) bit is a 0 if frame mode is to be used and a 1 iffield coding is to be used. Another embodiment is that the frame/fieldflag (112) bit is a 1 if frame mode is to be used and a 0 if fieldcoding is to be used.

Another embodiment of the present invention entails a method ofdetermining the size of blocks into which the encoder divides amacroblock in macroblock level AFF. A preferable, but not exclusive,method for determining the ideal block size is sum absolute difference(SAD) with or without bias or rate distortion (RD) basis. For example,SAD checks the performance of the possible block sizes and chooses theideal block size based on its results. The exact method of using SADwith or without bias or RD basis can be easily be performed by someoneskilled in the art.

According to an embodiment of the present invention, each frame andfield based macroblock in macroblock level AFF can be intra coded orinter coded. In intra coding, the macroblock is encoded withouttemporally referring to other macroblocks. On the other hand, in intercoding, temporal prediction with motion compensation is used to code themacroblocks.

If inter coding is used, a block with a size of 16 by 16 pixels, 16 by 8pixels, 8 by 16 pixels, or 8 by 8 pixels can have its own referencepictures. The block can either be a frame or field based macroblock. TheMPEG-4 Part 10 AVC/H.264 standard allows multiple reference picturesinstead of just two reference pictures. The use of multiple referencepictures improves the performance of the temporal prediction with motioncompensation algorithm by allowing the encoder to find a block in thereference picture that most closely matches the block that is to beencoded. By using the block in the reference picture in the codingprocess that most closely matches the block that is to be encoded, thegreatest amount of compression is possible in the encoding of thepicture. The reference pictures are stored in frame and field buffersand are assigned reference frame numbers and reference field numbersbased on the temporal distance they are away from the current picturethat is being encoded. The closer the reference picture is to thecurrent picture that is being stored, the more likely the referencepicture will be selected. For field mode coding, the reference picturesfor a block can be any top or bottom field of any of the referencepictures in the reference frame or field buffers.

Each block in a frame or field based macroblock can have its own motionvectors. The motion vectors are spatially predictive coded. According toan embodiment of the present invention, in inter coding, predictionmotion vectors (PMV) are also calculated for each block. The algebraicdifference between a block's PMVs and its associated motion vectors isthen calculated and encoded. This generates the compressed bits formotion vectors.

FIG. 12 will be used to explain various preferable methods ofcalculating the PMV of a block in a macroblock. A current block, E, inFIG. 12 is to be inter coded as well as its neighboring blocks A, B, C,and D. E will refer hereafter to a current block and A, B, C, and D willrefer hereafter to E's neighboring blocks, unless otherwise denoted.Block E's PMV is derived from the motion vectors of its neighboringblocks. These neighboring blocks in the example of FIG. 12 are A, B, C,and D. One preferable method of calculating the PMV for block E is tocalculate either the median of the motion vectors of blocks A, B, C, andD, the average of these motion vectors, or the weighted average of thesemotion vectors. Each of the blocks A through E can be in either frame orfield mode.

Another preferable method of calculating the PMV for block E is to use ayes/no method. Under the principles of the yes/no method, a block has tobe in the same frame or field coding mode as block E in order to haveits motion vector included in the calculation of the PMV for E. Forexample, if block E in FIG. 12 is in frame mode, block A must also be inframe mode to have its motion vector included in the calculation of thePMV for block E. If one of E's neighboring blocks does not have the samecoding mode as does block E, its motion vectors are not used in thecalculation of block E's PMV.

The “always method” can also be used to calculate the PMV for block E.In the always method, blocks A, B, C, and D are always used incalculating the PMV for block E, regardless of their frame or fieldcoding mode. If E is in frame mode and a neighboring block is in fieldmode, the vertical component of the neighboring block is multiplied by 2before being included in the PMV calculation for block E. If E is infield mode and a neighboring block is in frame mode, the verticalcomponent of the neighboring block is divided by 2 before being includedin the PMV calculation for block E.

The “selective method” can also be used to calculate the PMV for block Eif the macroblock has been encoded using pair based AFF encoding orgroup based AFF encoding. In the selective method, a frame-based blockhas a frame-based motion vector pointing to a reference frame. The blockis also assigned a field-based motion vector pointing to a referencefield. The field-based motion vector is the frame-based motion vector ofthe block with the vertical motion vector component divided by two. Thereference field number is the reference frame number multiplied by two.A field-based block has a field-based motion vector pointing to areference field. The block is also assigned a frame-based motion vectorpointing to a reference frame. The frame-based motion vector is thefield-based motion vector of the block with the vertical motion vectorcomponent multiplied by two. The reference frame number is the referencefield number divided by two.

The derivation of a block's PMV using the selective method will now beexplained using FIG. 12 as a reference. In macroblock pair based AFF,each block in a macroblock is associated with a companion block thatresides in the same geometric location within the second macroblock ofthe macroblock pair. In FIG. 12, each of block E's neighboring blocks(A, B, C, and D) may or may not be in the same frame or field codingmode as block E. Hence, the following rules apply:

If E is in frame mode and a neighboring block is in frame mode, the trueframe-based motion vector of the neighboring block is used for E's PMV.

If E is in frame mode and a neighboring block is in field mode, thefollowing rules apply in calculating E's PMV. If the neighboring block(e.g.; block A) and its companion field-based block have the samereference field, the average of the assigned frame-based motion vectorsof the two blocks is used for the calculation of E's PMV. The referenceframe number used for the PMV calculation is the reference field numberof the neighboring block divided by two. However, if the neighboringblock and its companion field block have different reference fields,then the neighboring block cannot be used in the calculation of E's PMV.

If E is in field mode and a neighboring block is in frame mode, thefollowing rules apply in calculating E's PMV. If the neighboring block(e.g.; block A) and its companion frame-based block have the samereference frame, the average of the assigned field-based motion vectorsof the two blocks is used for the calculation of E's PMV. The referencefield number used for the PMV calculation is the reference frame numberof the neighboring block multiplied by two. However, if the neighboringblock and its companion field block have different reference frames,then the neighboring block cannot be used in the calculation of E's PMV.

If E is in field mode and a neighboring block is in field mode, the truefield-based motion vector of the neighboring block is used in thecalculation of E's PMV.

An alternate preferable option can be used in the selective method tocalculate a block's PMV. In FIG. 12, each of block E's neighboringblocks (A, B, C, and D) may or may not be in the same frame or fieldcoding mode as block E. Hence, the following rules apply for thisalternate preferable option of the selective method:

If E is in frame mode and a neighboring block is in frame mode, the trueframe-based motion vector of the neighboring block is used for E's PMV.

If E is in frame mode and a neighboring block is in field mode, theweighted average of the assigned frame-based motion vectors of theneighboring block and its companion field-based block is used for thecalculation of E's PMV. The weighting factors are based upon thereference field numbers of the neighboring block and its companionblock.

If E is in field mode, and a neighboring block is in frame mode, theweighted average of the assigned field-based motion vectors of theneighboring block and its companion frame-based block is used for thecalculation of E's PMV. The weighting factors are based upon thereference frame numbers of the neighboring block and its companionblock.

If E is in field mode and a neighboring block is in field mode, the truefield-based motion vector of the neighboring block is used in thecalculation of E's PMV.

Another preferable method of computing a block's PMV is the “altselective method.” This method can be used in single macroblock AFFcoding, pair based macroblock AFF coding, or group based AFF coding. Inthis method, each block is assigned a horizontal and a vertical indexnumber, which represents the horizontal and vertical coordinates of theblock. Each block is also assigned a horizontal and vertical fieldcoordinate. A block's horizontal field coordinate is same as itshorizontal coordinate. For a block in a top field macroblock, thevertical field coordinate is half of vertical coordinate of the blockand is assigned top field polarity. For a block in the bottom fieldmacroblock, the vertical field coordinate of the block is obtained bysubtracting 4 from the vertical coordinate of the block and dividing theresult by 2. The block is also assigned bottom field polarity. Theresult of assigning different field polarities to two blocks is thatthere are now two blocks with the same horizontal and vertical fieldcoordinates but with differing field polarities. Thus, given thecoordinates of a block, the field coordinates and its field polarity canbe computed and vice versa.

The alt selective method will now be explained in detail using FIG. 12as a reference. The PMV of block E is to be computed. Let bx representthe horizontal size of block E divided by 4, which is the size of ablock in this example. The PMVs for E are obtained as follows dependingon whether E is in frame/field mode.

Let block E be in frame mode and let (x,y) represent the horizontal andvertical coordinates respectively of E. The neighboring blocks of E aredefined in the following manner. A is the block whose coordinates are(x−1,y). B is the block whose coordinates are (x,y−1). D is the blockwhose coordinates are (x−1,y−1). C is the block whose coordinates are(x+bx+1,y−1). If either A, B, C or D is in field mode then its verticalmotion vector is multiplied by 2 before being used for prediction andits reference frame number is computed by dividing its reference fieldby 2.

Now, let block E be in top or bottom field mode and let (xf,yf)represent the horizontal and vertical field coordinates respectively ofE. In this case, the neighbors of E are defined as follows. A is theblock whose field coordinates are (xf−1,yf) and has same polarity as E.B is the block whose field coordinates are (xf,yf−1) and has samepolarity as E. D is the block whose field coordinates are (xf−1,yf−1)and has same polarity as E. C is the block whose field coordinates are(xf+bx+1,yf) and has same polarity as E. If either A,B,C or D is inframe mode then its vertical motion vector is multiplied by 2 beforebeing used for prediction and its reference field is computed bymultiplying its reference frame by 2.

In all of the above methods for determining the PMV of a block, ahorizontal scanning path was assumed. However, the scanning path canalso be a vertical scanning path. In this case, the neighboring blocksof the current block, E, are defined as shown in FIG. 13. A verticalscanning path is preferable in some applications because the informationon all the neighboring blocks is available for the calculation of thePMV for the current block E.

Another embodiment of the present invention is directional segmentationprediction. In directional segmentation prediction, 16 by 8 pixel blocksand 8 by 16 pixel blocks have rules that apply to their PMV calculationsonly. These rules apply in all PMV calculation methods for these blocksizes. The rules will now be explained in detail in connection with FIG.12. In each of these rules, a current block E is to have its PMVcalculated.

First, a 16 by 16 pixel block consists of an upper block and a lowerblock. The upper block contains the top 8 rows of 16 pixels. The lowerblock contains the bottom 8 rows of 16 pixels. In the followingdescription, block E of FIG. 12 is a 16 by 8 pixel block. For the upperblock having a 16 by 8 pixel block, block B is used to predict block E'sPMV if it has the same reference picture as block E. Otherwise, medianprediction is used to predict block E's PMV. For the lower block havinga 16 by 8 pixel block, block A is used to predict block E's PMV if ithas the same reference picture as block E. Otherwise, median predictionis used to predict block E's PMV.

A 16 by 16 pixel block is divided into a right and left block. Bothright and left blocks are 8 by 16 pixels. In the following description,block E of FIG. 12 is a 8 by 16 pixel block. For the left block, block Ais used to predict block E's PMV if it has the same reference picture asblock E. Otherwise, median prediction is used to predict block E's PMV.For the right block, block C is used to predict block E's PMV if it hasthe same referenced picture as block E. Otherwise median prediction isused to predict block E's PMV.

For both 16 by 8 pixel blocks and 8 by 16 blocks, A, B, or C can be indifferent encoding modes (frame or field) than the current block E. Thefollowing rules apply for both block sizes. If E is in frame mode, andA, B, or C is in field mode, the reference frame number of A, B, or C iscomputed by dividing its reference field by 2. If E is in field mode,and A, B, or C is in frame mode, the reference field number of A, B, orC is computed by multiplying its reference frame by 2.

According to another embodiment of the present invention, a macroblockin a P picture can be skipped in AFF coding. If a macroblock is skipped,its data is not transmitted in the encoding of the picture. A skippedmacroblock in a P picture is reconstructed by copying the co-locatedmacroblock in the most recently coded reference picture. The co-locatedmacroblock is defined as the one with motion compensation using PMV asdefined above or without motion vectors. The following rules apply forskipped macroblocks in a P picture. If AFF coding is performed permacroblock, a skipped macroblock is in frame mode. If AFF coding isperformed on macroblock pairs and if both macroblocks are skipped, thenthey are in frame mode. However, if only one of the macroblocks in amacroblock pair is skipped, its frame or field coding mode is the sameas the non-skipped macroblock in the same macroblock pair. If AFF codingis performed on a group of macroblocks and if the entire group ofmacroblocks is skipped, then all the macroblocks are in frame mode. Ifthere is at least one macroblock that is not skipped, then the skippedmacroblocks in the same group are in the same frame or field coding modeas the non-skipped macroblock.

An alternate method for skipped macroblocks is as follows. If amacroblock pair is skipped, its frame and field coding mode follows itsneighboring macroblock pair to the left. If the left neighboringmacroblock pair is not available, its coding mode follows itsneighboring macroblock pair to the top. If neither the left nor topneighboring macroblock pairs are available, the skipped macroblock isset to frame mode.

Another embodiment of the present invention is direct mode macroblockcoding for B pictures. In direct mode coding, a B picture has two motionvectors, forward and backward motion vectors. Each motion vector pointsto a reference picture. Both the forward and backward motion vectors canpoint in the same temporal direction. For direct mode macroblock codingin B pictures, the forward and backward motion vectors of a block arecalculated from the co-located block in the backward reference picture.The co-located block in the backward reference picture can be frame modeor field mode coded. The following rules apply in direct mode macroblockcoding for B picture.

If the co-located block is in frame mode and if the current direct modemacroblock is also in frame mode, the two associated motion vectors of ablock in the direct mode macroblock are calculated from the co-locatedblock. The forward reference frame is the one used by the co-locatedblock. The backward reference frame is the same frame where theco-located block resides.

If the co-located block is in frame mode and if the current direct modemacroblock is in field mode, the two associated motion vectors of ablock in the direct mode macroblock are calculated from the co-locatedblock's motion vector with vertical component divided by two. Theforward reference field is the same parity field of the reference frameused by the co-located block. The backward reference field is the sameparity field of the backward reference frame where the co-located blockresides.

If the co-located block is in field mode and if the current direct modemacroblock is also in field mode, the two associated motion vectors of ablock in the direct mode macroblock are calculated from the co-locatedblock of the same field parity. The forward reference field is the fieldused by the co-located block. The backward reference field is the samefield where the co-located block resides.

If the co-located block is in field mode and if the current direct modemacroblock is in frame mode, the two associated motion vectors of theblock in the direct mode macroblock are calculated from the co-locatedblock's motion vector with vertical component multiplied by two. Theforward reference frame is the frame one of whose fields is used by theco-located block. The backward reference field is the frame in one ofwhose fields the co-located block resides.

An alternate option is to force the direct mode block to be in the sameframe or field coding mode as the co-located block. In this case, if theco-located block for a direct mode block is in frame mode, the directmode block is in frame mode as well. The two frame-based motion vectorsof the direct mode block are derived from the frame-based forward motionvector of the co-located block. The forward reference frame is used bythe co-located block. The backward reference frame is where theco-located block resides.

However, if the co-located block for a block in direct mode is in fieldmode, the direct mode block is also in field mode. The two field-basedmotion vectors of the direct mode block are derived from the field-basedforward motion vector of the co-located block. The forward referencefield is used by the co-located block. The backward reference field iswhere the co-located block resides.

A macroblock in a B picture can also be skipped in AFF coding accordingto another embodiment of the present invention. A skipped macroblock ina B picture is reconstructed as a regular direct mode macroblock withoutany coded transform coefficient information. For skipped macroblocks ina B picture, the following rules apply. If AFF coding is performed permacroblock, a skipped macroblock is either in frame mode or in the frameor field coding mode of the co-located block in its backward referencepicture. If AFF coding is performed on macroblock pairs and if bothmacroblocks are skipped, then they are in frame mode or in the frame orfield coding mode of the co-located macroblock pair in the its backwardreference picture. However, if only one of the macroblocks in amacroblock pair is skipped, its frame or field coding mode is the sameas the non-skipped macroblock of the same macroblock pair. If AFF codingis performed on a group of macroblocks and if the entire group ofmacroblocks is skipped, then all the macroblocks are in frame mode or inthe frame or field coding mode of the co-located group of macroblocks inthe backward reference picture. If there is at least one macroblock thatis not skipped, then the skipped macroblock in the same group are in thesame frame or field coding mode as the non-skipped macroblock.

As previously mentioned, a block can be intra coded. Intra blocks arespatially predictive coded. There are two possible intra coding modesfor a macroblock in macroblock level AFF coding. The first is infra 4×4mode and the second is intra_(—)16×16 mode. In both, each pixel's valueis predicted using the real reconstructed pixel values from neighboringblocks. By predicting pixel values, more compression can be achieved.The intra_(—)4×4 mode and the intra_(—)16×16 modes will each beexplained in more detail below.

For intra_(—)4×4 mode, the predictions of the pixels in a 4 by 4 pixelblock, as shown in FIG. 14, are derived form its left and above pixels.In FIG. 14, the 16 pixels in the 4 by 4 pixel block are labeled athrough p. Also shown in FIG. 14 are the neighboring pixels A through P.The neighboring pixels are in capital letters. As shown in FIG. 15,there are nine different prediction directions for intra_(—)4×4 coding.They are vertical (0), horizontal (1), DC prediction (mode 2), diagonaldown/left (3), diagonal down/right (4), vertical-left (5),horizontal-down (6), vertical-right (7), and horizontal-up (8). DCprediction averages all the neighboring pixels together to predict aparticular pixel value.

However, for intra_(—)16×16 mode, there are four different predictiondirections. Prediction directions are also referred to as predictionmodes. These prediction directions are vertical prediction (0),horizontal prediction (1), DC prediction, and plane prediction. Planeprediction will not be explained.

An intra block and its neighboring blocks may be coded in frame or fieldmode. Intra prediction is performed on the reconstructed blocks. Areconstructed block can be represented in both frame and field mode,regardless of the actual frame or field coding mode of the block. Sinceonly the pixels of the reconstructed blocks are used for intraprediction, the following rules apply.

If a block of 4 by 4 pixels or 16 by 16 pixels is in frame mode, theneighboring pixels used in calculating the pixel value predictions ofthe block are in the frame structure. If a block of 4 by 4 pixels or 16by 16 pixels is in field mode, the neighboring pixels used incalculating the pixel value prediction of the block are in fieldstructure of the same field parity.

The chosen intra-prediction mode (intra_pred_mode) of a 4 by 4 pixelblock is highly correlated with the prediction modes of adjacent blocks.This is illustrated in FIG. 16 a. FIG. 16 a shows that A and B areadjacent blocks to C. Block C's prediction mode is to be established.FIG. 16 b shows the order of intra prediction information in thebitstream. When the prediction modes of A and B are known (including thecase that A or B or both are outside the slice) the most probableprediction mode (most_probable_mode) of C is given. If one of the blocksA or B is “outside” the most probable prediction mode is equal DCprediction (mode 2). Otherwise it is equal to the minimum of predictionmodes used for blocks A and B. When an adjacent block is coded by 16×16intra mode, prediction mode is DC prediction mode. When an adjacentblock is coded a non-intra macroblock, prediction mode is “mode 2: DCprediction” in the usual case and “outside” in the case of constrainedintra update.

To signal a prediction mode number for a 4 by 4 block first parameteruse_most_probable mode is transmitted. This parameter is represented by1 bit codeword and can take values 0 or 1. If use_most_probable_mode isequal to 1 the most probable mode is used. Otherwise an additionalparameter remaining_(—) mode_selector, which can take value from 0 to 7is sent as 3 bit codeword. The codeword is a binary representation ofremaining_mode_selector value. The prediction mode number is calculatedas:

if(remaining_mode_selector<most_probable mode)

intra_pred_mode=remaining_mode_selector;

else

intra_pred_mode=remaining_mode_selector+1;

The ordering of prediction modes assigned to blocks C is therefore themost probable mode followed by the remaining modes in the ascendingorder.

An embodiment of the present invention includes the following rules thatapply to intra mode prediction for an intra-prediction mode of a 4 by 4pixel block or an intra-prediction mode of a 16 by 16 pixel block. BlockC and its neighboring blocks A and B can be in frame or field mode. Oneof the following rules shall apply. FIGS. 16 a-b will be used in thefollowing explanations of the rules.

Rule 1: A or B is used as the neighboring block of C only if A or B isin the same frame/field mode as C. Otherwise, A or B is considered asoutside.

Rule 2: A and B are used as the neighboring blocks of C, regardless oftheir frame/field coding mode.

Rule 3: If C is coded in frame mode and has co-ordinates (x,y), then Ais the block with co-ordinates (x,y−1) and B is the block withco-ordinates (x−1,y). Otherwise, if C is coded as field and has fieldco-ordinates (xf,yf) then A is the block whose field coordinates are(xf,yf−1) and has same field polarity as C and B is the block whosefield coordinates are (xf−1,yf) and has same field polarity as C.

Rule 4: This rule applies to macroblock pairs only. In the case ofdecoding the prediction modes of blocks numbered 3, 6, 7, 9, 12, 13, 11,14 and 15 of FIG. 16 b, the above and the left neighboring blocks are inthe same macroblock as the current block. However, in the case ofdecoding the prediction modes of blocks numbered 1, 4, and 5, the topblock (block A) is in a different macroblock pair than the currentmacroblock pair. In the case of decoding the prediction mode of blocksnumbered 2, 8, and 10, the left block (block B) is in a differentmacroblock pair. In the case of decoding the prediction mode of theblock numbered 0, both the left and the above blocks are in differentmacroblock pairs. For a macroblock in field decoding mode theneighboring blocks of the blocks numbered 0, 1, 4, 5, 2, 8, and 10 shallbe defined as follows:

If the above macroblock pair (170) is decoded in field mode, then forblocks number 0, 1, 4 and 5 in the top-field macroblock (173), blocksnumbered 10, 11, 14 and 15 respectively in the top-field macroblock(173) of the above macroblock pair (170) shall be considered as theabove neighboring blocks to the current macroblock pair (171) as shownin FIG. 17 a. For blocks number 0, 1, 4 and 5 in the bottom-fieldmacroblock (174), blocks numbered 10, 11, 14 and 15 respectively in thebottom-field MB of the above macroblock pair (170) shall be consideredas the above neighboring blocks to the current macroblock pair (171), asshown in FIG. 17 a.

However, if the above macroblock pair (170) is decoded in frame modethen for blocks number 0, 1, 4 and 5 in the top-field macroblock (173),blocks numbered 10, 11, 14 and 15 respectively in the bottom-framemacroblock (176) of the above macroblock pair (170) shall be consideredas the above neighboring blocks to the current macroblock pair (171), asshown in FIG. 17 b. For blocks number 0, 1, 4 and 5 in the bottom-fieldmacroblock (174), blocks numbered 10, 11, 14 and 15 respectively in thebottom-frame macroblock (176) of the above macroblock pair (170) shallbe considered as the above neighboring blocks to the current macroblockpair (171), as shown inn FIG. 17 b.

If the left macroblock pair (172) is decoded in field mode, then forblocks number 0, 2, 8 and 10 in the top-field macroblock (173), blocksnumbered 5, 7, 13 and 15 respectively in the top-field macroblock (173)of the left macroblock pair (172) shall be considered as the leftneighboring blocks to the current macroblock pair (171) as shown in FIG.17 c. For blocks number 0, 2, 8 and 10 in the bottom-field macroblock(174), blocks numbered 5, 7, 13 and 15 respectively in the bottom-fieldmacroblock (174) of the left macroblock pair (172) shall be consideredas the left neighboring blocks to the current macroblock pair (171), asshown in FIG. 17 c.

If the left macroblock pair (172) is decoded in frame mode, then forblocks number 0, 2, 8 and 10 in the top-field macroblock (173), theblocks numbered 5, 7, 13 and 15 respectively in the top-frame macroblock(175) of the left macroblock pair (172) shall be considered as the leftneighboring blocks to the current macroblock pair (171), as shown inFIG. 17 d. For blocks number 0, 2, 8 and 10 in the bottom-fieldmacroblock (174), blocks numbered 5, 7, 13 and 15 respectively in thebottom-frame macroblock (176) of the left macroblock pair (172) shall beconsidered as the left neighboring blocks to the current macroblock pair(171), as shown in FIG. 17 d.

For macroblock pairs on the upper boundary of a slice, if the leftmacroblock pair (172) is in frame decoding mode, then the intra modeprediction value used to predict a field macroblock shall be set to DCprediction.

The preceding descriptions of intra coding and intra mode prediction canbe extended to adaptive block transforms.

Another embodiment of the present invention is that loop filtering isperformed on the reconstructed blocks. A reconstructed block can berepresented in either frame or field structure, regardless of theframe/field coding mode of the block. Loop (deblock) filtering is aprocess of weighted averaging of the pixels of the neighboring blocks.FIG. 12 will be used to explain loop filtering. Assume E of FIG. 12 is areconstructed block, and A, B, C and D are its neighboring reconstructedblocks, as shown in FIG. 12, and they are all represented in framestructure. Since A, B, C, D and E can be either frame- or field-coded,the following rules apply:

Rule 1: If E is frame-coded, loop filtering is performed over the pixelsof E and its neighboring blocks A B, C and D.

Rule 2: If E is field-coded, loop filtering is performed over thetop-field and bottom-field pixels of E and its neighboring blocks A B, Cand D, separately.

Another embodiment of the present invention is that padding is performedon the reconstructed frame by repeating the boundary pixels. Since theboundary blocks may be coded in frame or field mode, the following rulesapply:

Rule 1: The pixels on the left or right vertical line of a boundaryblock are repeated, if necessary.

Rule 2: If a boundary block is in frame coding, the pixels on the top orbottom horizontal line of the boundary block are repeated.

Rule 3: if a boundary block is in field coding, the pixels on the twotop or two bottom horizontal (two field) lines of the boundary block arerepeated alternatively.

Another embodiment of the present invention is that two-dimensionaltransform coefficients are converted into one-dimensional series ofcoefficients before entropy coding. The scan path can be either zigzagor non-zigzag. The zigzag scanner is preferably for progressivesequences, but it may be also used for interlace sequences with slowmotions. The non-zigzag scanners are preferably for interlace sequences.For macroblock AFF coding, the following options may be used:

Option 1: The zigzag scan is used for macroblocks in frame mode whilethe non-zigzag scanners are used for macroblocks in field coding.

Option 2: The zigzag scan is used for macroblocks in both frame andfield modes.

Option 3: The non-zigzag scan is used for macroblocks in both frame andfield modes.

The preceding description has been presented only to illustrate anddescribe embodiments of invention. It is not intended to be exhaustiveor to limit the invention to any precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching.

The foregoing embodiments were chosen and described in order toillustrate principles of the invention and some practical applications.The preceding description enables others skilled in the art to utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the following claims.

1-15. (canceled)
 16. A method of encoding a picture in an imagesequence, comprising: dividing said picture into a plurality of smallerportions; selectively encoding at least one of said plurality of smallerportions in frame coding mode and at least one of said plurality ofsmaller portions in field coding mode; and deriving at least two motionvectors and two associated reference pictures or fields pointed by saidtwo motion vectors for at least one current block of said at least oneof said plurality of smaller portions if said picture is a bi-predictivecoded picture.
 17. The method of claim 16, wherein said at least twomotion vectors are computed from a co-located block in one of said twoassociated reference pictures or fields.
 18. An apparatus for encoding apicture in an image sequence, comprising: an encoder which divides saidpicture into a plurality of smaller portions; derives at least twomotion vectors and two associated reference pictures or fields pointedby said two motion vectors for at least one current block of said atleast one of said plurality of smaller portions if said picture is abi-predictive coded picture; and selectively encodes at least one ofsaid plurality of smaller portions in frame coding mode and at least oneof said plurality of smaller portions in field coding mode.
 19. Theapparatus of claim 18, wherein said at least two motion vectors arecomputed from a co-located block in one of said two associated referencepictures or fields.
 20. A computer-readable medium having stored thereona plurality of instructions, the plurality of instructions includinginstructions which, when executed by a processor, cause the processor toperform the steps of a method for encoding a picture in an imagesequence, comprising of: dividing said picture into a plurality ofsmaller portions; selectively encoding at least one of said plurality ofsmaller portions in frame coding mode and at least one of said pluralityof smaller portions in field coding mode; and deriving at least twomotion vectors and two associated reference pictures or fields pointedby said two motion vectors for at least one current block of said atleast one of said plurality of smaller portions if said picture is abi-predictive coded picture.
 21. A method of decoding an encoded picturehaving a plurality of smaller portions from a bitstream, comprising:decoding at least one of a plurality of smaller portions in frame codingmode and at least one of said plurality of smaller portions in fieldcoding mode; obtaining at least two motion vectors and two associatedreference pictures or fields for at least one current block of saidplurality of smaller portions if said picture is a bi-predictive codedpicture; and using said plurality of decoded smaller portions and saidat least two motion vectors for at least one current block of saidplurality of smaller portions to construct a decoded picture.
 22. Themethod of claim 21, wherein said at least two motion vectors arecomputed from a co-located block in one of said two associated referencepictures or fields.
 23. The method of claim 21, wherein said at leasttwo reference pictures or fields are deriving from a co-located block.24. The method of claim 21, wherein said co-located block in one of saidtwo associated reference pictures or fields is encoding in said framecoding mode or said field coding mode.
 25. An apparatus of decoding anencoded picture having a plurality of smaller portions from a bitstream,comprising: a decoder which decodes at least one of a plurality ofsmaller portions in frame coding mode and at least one of said pluralityof smaller portions in field coding mode; obtains at least two motionvectors and two associated reference pictures or fields for at least onecurrent block of said plurality of smaller portions if said picture is abi-predictive coded picture; and utilizes said plurality of decodedsmaller portions and said at least two motion vectors for at least onecurrent block of said plurality of smaller portions to construct adecoded picture.
 26. The apparatus of claim 25, wherein said at leasttwo motion vectors are computed from a co-located block in one of saidtwo associated reference pictures or fields.
 27. The apparatus of claim25, wherein said at least two reference pictures or fields are derivingfrom a co-located block.
 28. The apparatus of claim 27, wherein saidco-located block in one of said two associated reference pictures orfields is encoding in said frame coding mode or said field coding mode.29. A computer-readable medium having stored thereon a plurality ofinstructions, the plurality of instructions including instructionswhich, when executed by a processor, cause the processor to perform thesteps of a method of decoding an encoded picture having a plurality ofsmaller portions from a bitstream, comprising: decoding at least one ofa plurality of smaller portions in frame coding mode and at least one ofsaid plurality of smaller portions in field coding mode; obtaining atleast two motion vectors and two associated reference pictures or fieldsfor at least one current block of said plurality of smaller portions ifsaid picture is a bi-predictive coded picture; and using said pluralityof decoded smaller portions and said at least two motion vectors for atleast one current block of said plurality of smaller portions toconstruct a decoded picture.